Shannon's theorem and the Shannon-Hartley theorem, which are well known to those skilled in the art of information and coding, heuristically state that upper limits on a transmission channel's capacity are constrained by signal-to-noise ratio and channel bandwidth. Moreover, noise constraints and bandwidth constraints can be traded-off against one another to achieve a given channel capacity. In other words, any constraints on data rate imposed by signal-to-noise ratio can be compensated by an appropriately large signal bandwidth, and vice-versa.
Conventional transmission systems often utilize frequency modulation techniques to communicate data over a transmission channel having a limited bandwidth. Such techniques necessitate the use of a modulator and a demodulator to convert data into frequency fluctuations of a carrier waveform and to retrieve such data. The conventional FM demodulators, which include phase locked loop circuits, frequency locked loop circuits, and integrators driven by zero-crossing detectors, typically require an information signal's bandwidth to exhibit less than a maximum deviation frequency from a carrier frequency. Typically, such deviation is around .+-.15% or .+-.30% of the carrier frequency. Moreover, conventional FM demodulators require many cycles of a carrier waveform to decode the modulating information. Thus, the practical requirements of conventional FM demodulators potentially cause carrier frequency to artificially limit channel capacity.
For many applications, such as when public airwaves are used to broadcast information, regulatory requirements and power constraints limit channel bandwidths and received signal-to-noise ratios so that such factors, rather than carrier frequency, actually define maximum data rates. However, in other applications, such as when data is being communicated over a long cable, although signal frequency bandwidths are still limited, larger signal frequency bandwidths relative to the carrier frequency can be used. Moreover, control of transmitted power means that signal-to-noise ratios can be more easily manipulated so that they are not limiting. Thus, transmission systems which utilize conventional demodulation techniques artificially limit data rates.
Conventional modulation circuits tend to complement the conventional demodulation techniques. As previously mentioned, conventional demodulation techniques tend to utilize many cycles of a carrier waveform to accurately recover encoded information from the carrier waveform. Likewise, conventional modulation circuits tend to require excessive carrier cycles before producing an accurate instantaneous frequency.
Specifically, in frequency shift keying (FSK) operations a carrier frequency is modulated between discrete instantaneous frequency values. However, conventional modulation circuits briefly output the carrier at instantaneous frequencies intermediate to the proper discrete frequencies before producing an accurate output frequency. Since conventional demodulators require many cycles of the carrier waveform to recover the encoded data, the briefly appearing intermediate frequencies pose no greater limitation than is imposed by the conventional demodulation circuit. However, when a demodulator recovers data from the carrier waveform more quickly, the generation of intermediate frequencies limits the maximum data rates achievable by the transmission system.